Fix: don't install duplicate styles on USO

content/install-hook-userstyles.js

@@ -130,8 +130,18 @@
       return;
     }
     onClick.processing = true;
-    (event.type.includes('Update') ? onUpdate() : onInstall())
+    doInstall()
-      .then(done, done);
+      .then(() => {
+        if (!event.type.includes('Update')) {
+          // FIXME: sometimes the button is broken i.e. the button sends
+          // 'install' instead of 'update' event while the style is already
+          // install.
+          // This triggers an incorrect install count but we don't really care.
+          return getResource(getMeta('stylish-install-ping-url-chrome'));
+        }
+      })
+      .catch(console.error)
+      .then(done);
     function done() {
       setTimeout(() => {
         onClick.processing = false;
@@ -139,26 +149,26 @@
     }
   }
 
-
+  function doInstall() {
-  function onInstall() {
+    let oldStyle;
-    return getResource(getMeta('stylish-description'))
+    return API.findStyle({
-      .then(name => saveStyleCode('styleInstall', name))
+      md5Url: getMeta('stylish-md5-url') || location.href
-      .then(() => getResource(getMeta('stylish-install-ping-url-chrome')));
+    }, true)
-  }
+      .then(_oldStyle => {
-
+        oldStyle = _oldStyle;
-
+        return oldStyle ?
-  function onUpdate() {
+          oldStyle.name :
-    return new Promise((resolve, reject) => {
+          getResource(getMeta('stylish-description'));
-      API.findStyle({
+      })
-        md5Url: getMeta('stylish-md5-url') || location.href
+      .then(name => {
-      }, true).then(style => {
+        const props = {};
-        saveStyleCode('styleUpdate', style.name, {id: style.id})
+        if (oldStyle) {
-          .then(resolve, reject);
+          props.id = oldStyle.id;
+        }
+        return saveStyleCode(oldStyle ? 'styleUpdate' : 'styleInstall', name, props);
       });
-    });
   }
 
-
   function saveStyleCode(message, name, addProps = {}) {
     const isNew = message === 'styleInstall';
     const needsConfirmation = isNew || !saveStyleCode.confirmed;